Forensic analytical aspects of homemade explosives containing grocery powders and hydrogen peroxide

Homemade explosives become a significant challenge for forensic scientists and investigators. In addition to well-known materials such as acetone peroxide trimer, black powder, or lead azides, perpetrators often produce more exotic and less recognized Homemade Explosives (HMEs). Mixtures of hydrogen peroxide with liquid fuels are widely acknowledged as powerful explosives. Interestingly, similar explosive properties are found in mixtures of numerous solid materials with H2O2. Notably, powdered groceries, such as coffee, tea, grounded spices, and flour, are particularly interesting to pyrotechnics enthusiasts due to their easy production using accessible precursors, which do not attract the attention of security agencies. H2O2-based HMEs may become a dangerous component of improvised explosive devices for terrorists and ordinary offenders. For the four most powerful mixtures—HMEs based on coffee, tea, paprika, and turmeric—molecular markers useful for identification using the GC–MS technique have been proposed. Furthermore, the observed time-dependent changes in mixtures of H2O2 with these food products were studied and evaluated as a potential method for assessing the age of the evidence and reconstructing timelines of crimes. The paper also discusses the usefulness of FT-IR spectroscopy for identifying H2O2-based HMEs.


Black tea
The gas chromatograms of methanolic extracts from black tea 42 before and after treatment with hydrogen peroxide are presented in Fig. 2. The major component (refer to Tables 2 and 3) of the black tea extract is caffeine (CA), with traces of theobromine, palmitic acid, oleic acid, linolenic acid, phytol, and mono-palmitin (1-or 2-isomer).Additionally, impurities from the product container materials and/or those introduced during tea production were identified, including 6-azabicyclo[3.2.1]octane, oleamide, and octyl acrylate (or 2-ethylhexyl acrylate).It is probable that 8-methoxycaffeine detected forms spontaneously in the methanolic extract.
Dimethyl-and methylparabanic acids are oxidation products of caffeine and theobromine, respectively [43][44] .Cleavage of the C=C bond leads to the formation of 6,10,14-trimethyl-2-pentadecanone (from phytol) 45 , nonanoic acid (from oleic acid and its esters), and hexanoic acid (from linoleic acid and its derivatives).Corresponding aldehydes in dimethyl acetal forms, i.e., 1,1-dimethoxyhexane and 1,1-dimethoxynonane, are present at low concentrations [46][47][48][49][50] .DMPA does not form in black tea during storage in contact with air, both in its dry and wet forms.The concentration of caffeine in black tea oxidized with H 2 O 2 decreases sharply; after 60 min, its concentration is about one-tenth of the initial values.After 1 week of contact time, this compound becomes undetectable.
DMPA concentration increases monotonically in a series of samples with tea/H 2 O 2 contact time ranging from 1 to 60 min but declines during one-week-long oxidation.Phytol undergoes rapid oxidation and is not detected in samples treated for 5 min.In contrast, its oxidation product, 6,10,14-trimethyl-2-pentadecanone is stable, and www.nature.com/scientificreports/its concentration remains almost constant in all oxidized tea samples.The presence of dimethylparabanic acid is the best marker for tea treatment with hydrogen peroxide, as this product forms in relatively high amounts.However, this parameter may be used only for fresh samples.For old black tea material, which may have been treated with hydrogen peroxide for an extended period, the following properties indicated its HME character: the absence of caffeine, phytol, and unsaturated fatty acids, and the presence of 6,10,14-trimethyl-2-pentadecanone and short-chain aliphatic acids (C 6 and C 9 ).The processes mentioned above were not observed for black tea samples stored in contact with air, in both dry and wet conditions.The only noticeable effect is the decrease of caffeine in tea washed with water.As a result, HPOM samples based on black tea can be easily distinguished from old and weathered tea residues.
The H 2 O 2 treatment time can only be determined within a limited range through GC analyses.For the studied material, the [DMPA]/([DMPA] + [CA]) ratio shows a strong correlation with the reaction time (refer to Fig. 3).Absolute concentration values of these substances are poor predictors, as both compounds dissolve quite well in water.Therefore, their concentration in solid particles used for analysis decreases if the sample is washed with water or rinsed by rain.However, estimating these losses of analytes is difficult.Since H 2 O 2 is used in HMEs in amounts significantly exceeding CA, the reaction can be approximated by a pseudo-first-order kinetic model.Due to further DMPA decomposition, the [DMPA]/([DMPA] + [CA]) ratio serves as a reliable predictor of the oxidation time for samples treated for 2 days or less.Unfortunately, there is no GC-MS marker suitable for determining the age of older samples.

COOH COOH
Linoleic acid Linolenic acid

Coffee
The TIC GC-MS chromatograms for untreated and H 2 O 2 -treated samples are presented in Fig. 4. Similar to black tea, the major component in the methanolic extract of coffee is caffeine (refer to Tables 3 and 4).Other minor compounds include fatty acids (palmitic, oleic, linoleic; free and as methyl esters formed spontaneously in methanol solution), monopalmitin, 4-vinylguaiacol, and 2,6-dimethylpyrazine. Traces of plant steroids, such as β-sitosterol, stigmasterol, and campesterol, were also detected 51,52 .Among artificial compounds, octyl acrylate and oleamide were identified.
In opposition to black tea, the oxidation of caffeine in coffee comes more slowly.This is likely due to the high lipid content in powdered coffee.Firstly, this increases the material's hydrophobicity, inhibiting contact with hydrogen peroxide solution.Moreover, unsaturated fatty acid moieties act as peroxide scavengers, slowing down  www.nature.com/scientificreports/ the oxidation of caffeine.As a result, the formation of dimethylparabanic acid (DMPA) is slow, and because this product undergoes subsequent reactions, DMPA does not accumulate in the sample and may be hard to detect.Small amounts of caffeine and DMPA are present even in coffee-based HPOM samples oxidized for over 1 week.Therefore, DMPA is a poor marker for HME mixtures based on the coffee/H 2 O 2 system.Other compounds found in oxidized coffee powder include citraconic anhydride, benzoic acid, pentanoic acid, nonanoic acid, and 2,4-decadienal (two isomers).
The best markers indicating that coffee has been mixed with concentrated hydrogen peroxide are the decreasing ratios of linolenic to palmitic acid and caffeine to palmitic acid (note that this parameter may be affected by caffeine washing out by water).In our samples, the first parameter reduces from 2.12 to 1.15 and the second from 0.65 to 0.42 in a 60-min reaction time.However, these parameters do not permit establishing reaction time since they exhibit high variation and low repeatability.
Changes discussed above do not occur in ground coffee during its storage in contact with air, even in wet samples; therefore, they are specific for coffee-based HPOMs.
The most exact markers for turmeric/H 2 O 2 -based HMEs are the absence of α-turmerone and only traces of curlone.For older samples, Ar-turmerone is also absent.On the other hand, the most characteristic markers of H 2 O 2 -oxidized samples, occurring in significant amounts, are coumaran and 4-vinylguaiacol.
The content of volatile components decreases slowly in turmeric during storage in open containers; however, even in one-year-old samples, sesquiterpenes are detected in high concentrations.α-Turmerone and curlone are air-sensitive, especially when the material is exposed to sunlight; therefore, their concentration decreases significantly over a few months, and these markers may not be present in weathered turmeric powder.Ar-turmerone is stable and does not decline in material stored in contact with air and/or water; therefore, its absence is a good marker for turmeric-based HPOM explosives.Also, above mentioned oxidation products (e.g., coumaran, 4-vinylguaiacol) do not form during turmeric storage, so their presence in the analyzed samples clearly indicates contact with H 2 O 2 .Decreasing Ar-turmerone and curlone concentrations may be used to estimate the contact time of the turmeric and hydrogen peroxide (refer to Fig. 6).These parameters differ in their application range.Ar-turmerone decays slowly, and its concentration may be reliably used for 1 day or older material, while curlone decreases within a few hours.There is no good marker for estimating a sample's age longer than a week.

Sweet paprika
The gas chromatogram of methanolic sweet paprika 58 extract is relatively scanty (refer to Fig. 7).The major components (see Tables 3 and 6) are free fatty acids, including myristic, palmitic, palmitoleic, linoleic, and stearic acids.As trace ingredients, one may find monoglycerides (monopalmitin, monolinolenin, and monolinolein), 2,4-dihydroxy-2,5-dimethyl-3(2H)-furan-3-one, 3,5-dihydroxy-6-methyl-2,3-dihydro-4H-pyran-4-one, levulinic acid, maltol, 1-monoacetin, and dihydroactinidiolide.After contact with H 2 O 2, several new components form, including 2-heptenal, 2-octenal, 2-decenal (two isomers), 4-decenal, 2,4-decadienal (two isomers), pentanoic acid, hexanoic acid and nonanoic acid 49 .The formation of these molecules is related to the cleavage of unsaturated fatty acid moieties.Additionally, traces of epoxylated terpenoid-β-ionone 5,6-epoxide were found in some samples.The most characteristic feature of H 2 O 2 -treated sweet paprika powder is the decreasing ratio of unsaturated fatty acids to total fatty acid content (from 0.89 ± 0.02 to 0.72 ± 0.05 in studied samples after 60 min of reaction).Another characteristic is the presence of unsaturated aldehydes, e.g., 2-heptenal; however, this parameter is less stable due to the volatility of these compounds.Unfortunately, the abovementioned parameters are unsuitable for estimating sample age or reaction time due to low repeatability.The ratio of unsaturated fatty acids to total fatty acids is a stable parameter in sweet paprika samples stored for a long time in contact with air or moisture.The exposure of the material to sunlight may result in a decrease in linoleic acid content; however, other fatty acids detectable in extracts remain stable under such condition.

Discussion
Dangerous and potent HMEs produced from powdered food products pose a challenging task for forensic scientists.However, GC-MS analyses allow for the detection of contact between grocery materials and hydrogen peroxide.The changes are particularly noticeable for some of the studied components (turmeric, black tea), while for others (coffee, sweet paprika), the changes are primarily quantitative.Changes detected in the former materials show clear time-dependency, enabling the determination of when the HME was prepared.However,   www.nature.com/scientificreports/among the studied materials, a suitable marker for age determination of older traces (approximately 1 week old) was identified only for turmeric.In general, studies of such mixtures should focus on the following stages: -Protection of the sample, washing out hydrogen peroxide, extraction, and GC analysis (preparation of the extract should be made as soon as possible); -Identification of oxidation markers-if present, HME production is highly probable; -Quantitative comparison of oxidation markers concentration with those in original, untreated material; -Reconstruction of the studied mixture and determination of the time dependence of HME markers' concentration for the estimation of the original sample age.
The current preliminary results indicate the usefulness of the GC-MS technique for studying HMEs based on groceries and hydrogen peroxide mixtures (HPOM).Methanolic extracts obtained from HPOM samples remain stable over time, except for the formation of methyl esters from fatty acids and acetals from aldehydes.These processes need to be considered during the results analysis.The discussed changes (formation of oxidation products and the decrease in the concentration of some components of the original groceries) also occur at low or elevated temperatures (e.g., during winter or summer heat); therefore, HPOMs prepared in real, non-laboratory conditions can be identified.However, the kinetics of the oxidation processes are influenced by temperature, necessitating reconstruction at the actual temperature in all cases.Further areas of study include the analysis of HMEs containing other powdered food products (e.g., black pepper, flour) and the application of different analytical techniques, especially HPLC and SPME-GC chromatography.

Methods
Caution! Concentrated hydrogen peroxide is corrosive to the skin.The mixtures described below warm up spontaneously and may ignite if prepared in larger amounts.Combinations of hydrogen peroxide with powdered groceries can violently explode when stimulated with a primary explosive!

Materials
Hydrogen peroxide pure (Warchem, PL), with a nominal concentration 60% w/w and an actual concentration 56 ± 1% w/w, was used without purification; the concentration was determined by the standard iodometric procedure.
Peroxides were detected in an aqueous solution using the peroxide strip test XploSens PS™ (Xplosafe LLC, USA).
General procedure 1 g of powdered grocery was mixed with 5 mL of hydrogen peroxide (56%).The samples were stored at room temperature (20 ± 1 °C).The reaction was quenched with water (20 mL), and the mixture was then centrifuged (4 000 rpm, 5 min).The solid fraction was washed several times with distilled water until a negative test for peroxides was obtained in the supernatant.The filtered-off solid material was dried in a desiccator over P 2 O 5 for 2 h under vacuum.Oxidation was quenched after 1, 5, 15, 30, and 60 min.An additional sample of material that remained in contact with hydrogen peroxide for over 1 week was also prepared.

FT-IR analyses
Transmission FT-IR spectra (4000-400 cm −1 ) were recorded using Bruker IFS 66v/S spectrometer.Samples were prepared as KBr pellets (1.5 mg of sample in 200 mg of KBr), and 256 scans were acquired.

GC-MS analyses
25 mg of solid sample was suspended in 1 mL of methanol, and 5 µL of n-dodecane (internal standard) was added.The mixture was then sonicated in an ultrasonic bath for over 10 min at room temperature.Subsequently, the suspension was filtered through a syringe filter (pore size 0.45 µm) and analyzed using a Varian 4000GC/MS gas chromatograph equipped with a VF-5 ms column (30 m × 0.25 mm, d f 0.25 µm; Agilent Technologies).The analysis parameters were as follows: carrier gas-helium, gas flow-1 ml/min, injector temperature-220 °C, column oven program-40 °C (isothermal, 3 min.),linear ramp 15 °C/min to 280 °C, 280 °C (isothermal, 10 min).Signals were identified using the NIST MS Search mass spectra library and literature data.Intensities are reported relative to the signal of internal standard (int.= 1.0).

Table 1 .
Major components of studied materials and characteristic IR bands.

Table 2 .
Components detected in black tea extracts.

Table 3 .
Fatty acids and their cleavage products detected in studied samples.

Table 4 .
Components detected in coffee extracts.

Table 5 .
Components detected in turmeric extracts.

Table 6 .
Components detected in paprika extracts.